1. Field of the Invention
The present invention relates to a technique for preventing the intrusion of external noise into an up signal on a signal transmission path which bidirectionally transmits a high frequency signal such as a bi-directional CATV system. More specifically, the present invention relates to an external noise intrusion prevention device which prevents the intrusion of external noise as well as a signal amplifier, a protector and an antenna plug each of which includes an external noise intrusion prevention circuit.
2. Description of Related Art
In a bi-directional CATV system, a frequency band of 70 MHz or higher is normally used for a down signal transmitted from a center station. A frequency band of 5 MHz to 42 MHz is used for a frequency of up signal transmitted from each subscriber's house to the center station.
The frequency band of the up signal is also a frequency band of noise produced in a home electric appliance or the like. For that reason, much noise is intruded into an up signal, which becomes a main cause for ingress noise in the center station. To reduce the ingress noise, a switching unit is conventionally provided in a trunk and, if necessary, a part of a trunk system having large noise is cut off.
However, the cutoff of the trunk system means that a line is mechanically cut off. As a result, a signal is intercepted, as well, which causes another problem. This prevention method cannot be, therefore, always used. Under these circumstances, demand for a technique which can reduce ingress noise itself, i.e., decrease or prevent the intrusion of external noise into the central conductor of a trunk cable without intercepting a signal rises.